1. Technical Field
The present invention relates to an electrostatic ultrasonic transducer that generates a high sound pressure of a fixed level over a wide range of frequency, an ultrasonic speaker using the same and an audio signal reproduction method using the electrostatic ultrasonic transducer, an electrode manufacturing method for use in an ultrasonic transducer, an ultrasonic transducer manufacturing method, a superdirective acoustic system, and a display device.
2. Related Art
Most of the previous ultrasonic transducers are of resonance type using a piezoelectric ceramic material.
FIG. 16 shows the configuration of the previous ultrasonic transducer. Most of the previous ultrasonic transducers are of resonance type using a piezoelectric ceramic material for an oscillator. Using the piezoelectric ceramic material for an oscillator, the ultrasonic transducer of FIG. 16 performs conversion from an electric signal to ultrasound and from ultrasound to an electric signal, i.e., performs transmission and reception of ultrasound. The ultrasonic transducer of FIG. 16 is of bimorph type, and is configured to include: two piezoelectric ceramic plates 61 and 62, a cone 63, a case 64, leads 65 and 66, and a screen 67.
The piezoelectric ceramic plates 61 and 62 are affixed together, and the leads 65 and 66 are respectively connected to their surfaces opposite to the affixed surfaces.
The ultrasonic transducer of resonance type makes use of a resonance phenomenon observed in the piezoelectric ceramic plates so that the characteristics in terms of ultrasound transmission and reception are increased in a relatively narrow range of frequency of a level closer to the resonance frequency.
Unlike such an ultrasonic transducer of resonance type shown in FIG. 16, the previous ultrasonic transducer of electrostatic type is known as an ultrasonic transducer of broadband oscillation type with which a high sound pressure can be generated over a wide range of frequency. Such an electrostatic ultrasonic transducer is referred to as of Pull type because the oscillation film moves only in the direction toward the side of a fixed electrode. FIG. 17 shows the specific configuration of such an ultrasonic transducer of broadband oscillation type (Pull type).
The electrostatic ultrasonic transducer of FIG. 17 is using a dielectric 131 (insulator) as an oscillator. The dielectric 131 is made of PET (polyethylene terephthalate resin) or others with a thickness of about 3 to 10 μm. On the upper surface portion of the dielectric 131, an upper electrode 132 is formed as a piece therewith by evaporation or others. The upper electrode 132 is formed as a metal leaf, e.g., aluminum leaf. A lower electrode 133 made of brass is so provided as to come in contact with the lower surface portion of the dielectric 131. The lower electrode 133 is connected with a lead 152 and is fixed to a base plate 135 made of Bakelite or others.
The upper electrode 132 is connected with a lead 153, which is connected to a direct-current (DC) bias power supply 150. The DC bias power supply 150 serves to always apply a DC bias voltage to the upper electrode 132. The DC bias voltage is of about 50 to 150V for absorbing the upper electrode so that the upper electrode 132 is absorbed to the side of the lower electrode 133. A reference numeral 151 denotes a signal source.
The components of the dielectric 131, the upper electrode 132, and the base plate 135 are swaged by a case 130 together with a metal rings 136, 137, and 13S, and a mesh 139.
The surface of the lower electrode 133 on the side of the dielectric 131 is formed with a plurality of small randomly-shaped grooves of about several tens to hundreds of micrometers (μm). These small grooves each serve as a gap between the lower electrode 133 and the dielectric 131 so that a small change is observed thereby in the distribution of the capacitance between the upper and lower electrodes 132 and 133.
These randomly-shaped small grooves are formed by manually filing the surface of the lower electrode 133 to make it rough. By forming such an unlimited number of capacitors varying in gap size and depth, in the ultrasonic transducer of electrostatic type, the frequency response of the ultrasonic transducer of FIG. 17 covers a wide range of frequency as indicated by a curve Q1 of FIG. 18.
With the ultrasonic transducer of the configuration as above, a square wave signal (50 to 150 Vp-p) is applied between the upper and lower electrodes 132 and 133 in the state that the upper electrode 132 is being applied with the DC bias voltage. For information, as indicated by a curve Q2 of FIG. 18, the frequency response of an ultrasonic transducer of resonance type is −30 dB with respect to the maximum sound pressure with an assumption that the central frequency, i.e., resonance frequency of the piezoelectric ceramic plate, is, for example, 40 kHz, and covers the frequency of ±5 kHz with respect to the central frequency leading to the maximum sound pressure.
As to the ultrasonic transducer of broadband oscillation type configured as above, the frequency response thereof is flat in a range from 40 kHz to 100 kHz or so, and is about ±6 dB with 100 kHz compared with the maximum sound pressure. For details, refer to Patent Document 1 (JP-A-2000-50387), and Patent Document 2 (JP-A-2000-50392).
As described above, unlike the ultrasonic transducer of resonance type shown in FIG. 16, the ultrasonic transducer of electrostatic type shown in FIG. 17 is previously known as an ultrasonic transducer of broadband oscillation type (Pull type) with which a relatively high sound pressure can be generated over a wide range of frequency. The problem with such an electrostatic ultrasonic transducer is that, as shown in FIG. 18, the maximum sound pressure is low as 120 dB or lower compared with the sound pressure of 130 dB or higher for the ultrasonic transducer of resonance type. The sound pressure is thus somewhat not enough for use as an ultrasonic speaker.
A description is now given for the ultrasonic speaker. In the ultrasonic speaker, a signal in an ultrasonic frequency band called carrier wave is subjected to AM (amplitude) modulation using an audio signal, i.e., signal in an audio frequency band. The resulting modulation signal is used to drive the ultrasonic transducer, whereby the acoustic wave is emitted into the air. The acoustic wave is in the state that the ultrasound is modulated by an audio signal from a signal source. With such acoustic wave emission, the original audio signal is self-reproduced in the air due to the nonlinearity of the air.
That is, because the acoustic wave is a compressional wave being transmitted with a medium of air, in the course of transmission, the modulated ultrasound is affected by the apparent difference of air density, i.e., the air is dense in some portion, and not in dense in other portions. This causes an increase of the speed of sound in the air-dense portion, and a decrease in the no-air-dense portion. As a result, the modulation wave itself suffers from distortion, and is subjected to curve fitting to derive a carrier wave (ultrasound) and an audible wave (original audio signal). As such, we human beings can hear only the audible sound of 20 kHz or lower, i.e., only the original audio signal, and such a principle is generally referred to as the parametric array effects.
To maximize the parametric array effects, the ultrasound pressure of 120 dB or higher is required. The electrostatic ultrasonic transducer, however, has a difficulty in achieving this value, and thus a ceramic piezoelectric device made of PZT (lead zirconate titanate) or a polymer piezoelectric device made of PVDF (polyvinylidene fluoride) has been used as an ultrasound transmitter.
The issue here is that a resonance point of the piezoelectric device forms a sharp angle irrespective of the material, and the resonance frequency is used for driving so that the ultrasonic speaker is put in practical use. In this sense, the frequency domain that can ensure the high sound pressure is considerably narrow, i.e., is of narrow bandwidth.
The maximum frequency bandwidth audible for human beings is generally understood as 20 Hz to 20 kHz, and has a bandwidth of about 20 kHz. That is, if with an ultrasonic speaker, it is impossible to demodulate the original audio signal with fidelity unless the high sound pressure is kept over the frequency range of 20 kHz in the ultrasonic domain. For the previous ultrasonic speaker of resonance type using a piezoelectric device, it is easily understood that there is hardly a chance to reproduce (demodulate) such a broadband as 20 kHz with fidelity.
Actually, there have been problems with an ultrasonic speaker using the previous ultrasonic speaker of resonance type, e.g., narrow bandwidth with bad audio reproduction quality, the maximum degree of modulation is of about 0.5 because if the degree of AM modulation is increased too much, the demodulated audio will sound distorted, increasing an input voltage (increasing volume) causes unstable oscillation of a piezoelectric device and the audio thus sounds raspy, and a higher voltage easily damages the piezoelectric device, and difficulties in array configuration, size increase, and size reduction, thereby resulting in cost increase.
An ultrasonic speaker using the electrostatic ultrasonic transducer (Pull type) of FIG. 17 can nearly solve the problems of the previous technology. Although the ultrasonic speaker can cover a wide range of bandwidth, there is still a problem of the shortage of absolute sound pressure to derive sufficient volume for the demodulated audio.
The electrostatic ultrasonic transducer of Pull type has also a problem if it is used for an ultrasonic speaker. That is, the electrostatic force acts only in the direction toward the side of a fixed electrode, and thus an oscillation film (corresponding to the upper electrode 132 of FIG. 17) does not oscillate symmetrically. Therefore, in such a case, the oscillation of the oscillation film directly generates audio sound.
In consideration thereof, for the purpose of deriving the parametric array effects over a wide range of frequency, we inventors have already proposed an electrostatic ultrasonic transducer that can generate an acoustic signal being sufficiently high in sound pressure level. In this electrostatic ultrasonic transducer, an oscillation film with a conductor layer is sandwiched between a pair of fixed electrodes each formed with through holes at their opposing positions. The pair of fixed electrodes are applied with an alternating signal in the state that the oscillation film is being applied with the DC bias voltage.
Such an electrostatic ultrasonic transducer is called electrostatic ultrasonic transducer of Push-Pull type, in which an oscillation film sandwiched between a pair of fixed electrodes receives the electrostatic suction force and the electrostatic repulsive force at the same time in the same direction corresponding to the polarity of the alternating signal. This not only allows to sufficiently increase the oscillation amplitude of the oscillation film to derive the parametric array effects but also to keep the oscillation symmetrical so that the transducer of Push-Pull type can generate the higher sound pressure over a wide range of frequency compared with the previous electrostatic ultrasonic transducer of Pull type.
Even with such an electrostatic ultrasonic transducer of Push-Pull type, however, if no change is made thereto, there is still a difficulty in generating the sufficient level of sound pressure in the air. This is because the through holes are relatively small in size to make the sound pass therethrough.
As such, even the electrostatic ultrasonic transducer of such Push-Pull type is not yet enough to generate the sufficient level of sound pressure.